104 



PHOTO- AND CHEMOSYNTHESIS OF BACTERIA 



CHAP. 5 



photosynthesis : 



(5.3) CO2 + I H2O + i S 



(5.4) 



(5.5) 



3- ...V. , 3 > {CHaO! 4- I H+,. + I (S04)aq- - 14 kcal 



CO2 + I H2O + I (S203)aq-. > {CH2OI + (HS04)aq. " 6 kcal 



CO2 + H2O + 2 (HS03)aq. • ^ ICH2OI + 2 (HS04)aq. + 17 kcal 



A form of bacterial photosynthesis particularly suitable for quanti- 

 tative study, is the carbon dioxide-hydrogen assimilation which produces 

 only organic matter and water. According to the equation: 



(5.6) CO2 + 2H2 > {CHsOl + H2O + 25.1 kcal 



the quotient AH2/ACO2 should be equal to 2. 



Reaction (5.6) was discovered by Roelefson (1934) in the study of 

 Thiorhodaceae. In the same year, Gaffron found, that certain Athio- 

 rhodaceae also can reduce carbon dioxide by means of molecular hydrogen 

 in light. Table 5. Ill contains several determinations of the "photo- 



Table 5.III 

 Photosynthetic Quotients of Hydrogen-Consuming Bacteria 



Organism 



Athiorhodaceae 



Rhodovibrio parvus 

 Streptococcus varians 

 Streptococcus varians 

 Thiorhodaceae: Chromatium sp. 



AH2/ACO2 



1.85-2.25 

 2.2 -2.6 

 2.6 



2.4 



Observer 



Gaffron (1935) 

 van Niel (1941) 

 Wessler and French (1939) 

 van Niel (1936) 



synthetic quotient" AH2/ACO2. Most values in the table are somewhat 

 larger than 2, indicating a possible formation of products reduced beyond 

 the carbohydrate stage. 



We have given, in equations (5.2) to (5.6), the heats of the photo- 

 reduction of one mole of carbon dioxide by bacteria (calculated from the 

 data of Bichowsky and Rossini, assuming 51 kcal for the heat of formation 

 of the {CH2O} group). They vary between Ai7 =4-13 kcal for the oxi- 

 dation of sulfur to sulfuric acid, and AH = — 25 kcal for the reduction 

 of carbon dioxide by molecular hydrogen. Thus, the photochemical 

 reactions of autotrophic bacteria are either exothermal, or only weakly 

 endothermal (as compared with the photosynthesis of the higher plants, 

 AH =112 kcal). However, the reduction of carbon dioxide by ele- 

 mentary selenium (Sapozhnikov) should involve the accumulation of as 

 much as 60 kcal per mole (if selenium is oxidized to selenic acid). Fur- 

 thermore, according to Eymers and Wassink (1938) the carbon dioxide 

 reduction by thiosulfate in Chromatium D leads to the oxidation of the 

 latter to tetrathionate, a strongly endothermal reaction. Using the heats 

 of formation of the ions given by Bichowsky and Rossini, we obtain: 



(5.7) CO2 + 4 (S203)aq- + 3 H2O > ICH20} + 2 (S406)aq-. + 



4 (OH)aq. - 68 kcal 



